1. Field of the Invention
The present invention relates to heat dissipation devices and methods for fabricating the same. In particular, the present invention relates to a multiple step injection molding technique used to form a heat dissipation device comprising at least two separate conductive material regions.
2. State of the Art
Higher performance, lower cost, increased miniaturization of integrated circuit components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. As these goals are achieved, microelectronic dice become smaller. Accordingly, the density of power consumption of the integrated circuit components in the microelectronic die has increased, which, in turn, increases the average junction temperature of the microelectronic die. If the temperature of the microelectronic die becomes too high, the integrated circuits of the microelectronic die may be damaged or destroyed.
Various apparatus and techniques have been used and are presently being used for removing heat from microelectronic dice. One such heat dissipation technique involves the attachment of a high surface area heat sink to a microelectronic die. FIG. 12 illustrates an assembly 300 comprising a microelectronic die 302 (illustrated as a flip chip) physically and electrically attached to a substrate carrier 304 by a plurality of solder balls 306. A heat sink 308 is attached to a back surface 312 of the microelectronic die 302 by a thermally conductive adhesive 314. The heat generated by the microelectronic die 302 is drawn into the heat sink 308 (following the path of least thermal resistance) by conductive heat transfer.
High surface area heat sinks 308 are generally used because the rate at which heat is dissipated from a heat sink is substantially proportional to the surface area of the heat sink. The high surface area heat sink 308 usually includes a plurality of projections 316 extending substantially perpendicularly from the microelectronic die 302. It is, of course, understood that the projections 316 may include, but are not limited to, elongate planar fin-like structures and columnar/pillar structures. The high surface area of the projections 316 allows heat to be convectively dissipated from the projections 316 into the air surrounding the high surface area heat sink 308. A fan 318 may be incorporated into the assembly 300 to enhance the convective heat dissipation.
The heat sinks 308 may be fabricated by molding, such as injection or extrusion, or by forming the projections 316 from a block of conductive material (such as by skiving) or attaching projections (such as folded fins) to a conductive block. Furthermore, the heat sinks 308 may be constructed from a thermally conductive material, such as copper, silver, gold, aluminum, and alloys thereof. However, although copper, gold, and silver have excellent thermal conductivity (e.g.;, greater than about 300 J/(s*m*xc2x0 C.) between about 0xc2x0 C. and 100xc2x0 C.), they are heavy (e.g., specific gravities of greater than about 8.0), such that the weight of the heat sink 308 could damage the microelectronic die 302 to which it is attached. Furthermore, they are expensive (prohibitively so with gold and silver) relative to other conductive materials. Thus, less expensive and lighter materials such as aluminum (i.e., a specific gravity of about 2.7) could be used. However, since aluminum and other lighter materials generally have lower thermal conductive properties lower than gold, silver, and copper (less than about 300 J/(s*m*xc2x0 C.) between about 0xc2x0 C. and 100xc2x0 C.), they may not have sufficient thermal conductive properties to adequately cool a high heat producing microelectronic die 302.
Thus, some heat sinks are a combination of highly thermally conductive materials and lightweight, relatively, less thermally conductive material to form multiple conductive material designs. FIG. 13 illustrates such a heat sink 320 comprising a highly thermally conductive plate portion 322 (such as copper) and a lightweight thermally conductive, high surface area portion 324 (such as aluminum) having projections 326 thereon. The plate portion 322 and the high surface area portion 324 are attached to one another by any known connection method. This design allows the highly thermally conductive plate portion 322 to thermally contact the microelectronic die 302 for effective heat removal and to conduct the heat to the lighter, high surface area portion 324 for convective dissipation to the surrounding air.
Another design of a heat sink 330 comprises an extruded, lightweight, high surface area portion 332 having a plurality of projections 334 and a highly conductive plate portion 336 which has been pressed into the high surface area portion 332, as shown in FIG. 14. Both multiple metal designs of FIGS. 13 and 14 result in lightweight heat sinks; however, the interface between the high surface area portions and the plate portions may not have an efficient contact. Surface variations between the high surface area portion and the plate portion may result in very small voids/air spaces, which reduces the efficiency of the thermal contact therebetween.
Therefore, it would be advantageous to develop techniques to fabricate a multiple material heat sink that has efficient thermal contact between the various materials in the heat sink.